README.md

Docker: the Linux container engine

Docker is an open source project to pack, ship and run any application
as a lightweight container

Docker containers are both hardware-agnostic and platform-agnostic.
This means they can run anywhere, from your laptop to the largest
EC2 compute instance and everything in between - and they don't require
you to use a particular language, framework or packaging system. That
makes them great building blocks for deploying and scaling web apps,
databases, and backend services without depending on a particular stack
or provider.

Docker began as an open-source implementation of the deployment engine which
powers dotCloud, a popular Platform-as-a-Service.
It benefits directly from the experience accumulated over several years
of large-scale operation and support of hundreds of thousands of
applications and databases.

Security Disclosure

Security is very important to us. If you have any issue regarding security,
please disclose the information responsibly by sending an email to
security@docker.com and not by creating a github issue.

Better than VMs

A common method for distributing applications and sandboxing their
execution is to use virtual machines, or VMs. Typical VM formats are
VMWare's vmdk, Oracle Virtualbox's vdi, and Amazon EC2's ami. In theory
these formats should allow every developer to automatically package
their application into a "machine" for easy distribution and deployment.
In practice, that almost never happens, for a few reasons:

Size: VMs are very large which makes them impractical to store
and transfer.

Performance: running VMs consumes significant CPU and memory,
which makes them impractical in many scenarios, for example local
development of multi-tier applications, and large-scale deployment
of cpu and memory-intensive applications on large numbers of
machines.

Portability: competing VM environments don't play well with each
other. Although conversion tools do exist, they are limited and
add even more overhead.

Hardware-centric: VMs were designed with machine operators in
mind, not software developers. As a result, they offer very
limited tooling for what developers need most: building, testing
and running their software. For example, VMs offer no facilities
for application versioning, monitoring, configuration, logging or
service discovery.

By contrast, Docker relies on a different sandboxing method known as
containerization. Unlike traditional virtualization, containerization
takes place at the kernel level. Most modern operating system kernels
now support the primitives necessary for containerization, including
Linux with openvz,
vserver and more recently
lxc, Solaris with
zones,
and FreeBSD with
Jails.

Docker builds on top of these low-level primitives to offer developers a
portable format and runtime environment that solves all four problems.
Docker containers are small (and their transfer can be optimized with
layers), they have basically zero memory and cpu overhead, they are
completely portable, and are designed from the ground up with an
application-centric design.

Perhaps best of all, because Docker operates at the OS level, it can still be
run inside a VM!

Plays well with others

Docker does not require you to buy into a particular programming
language, framework, packaging system, or configuration language.

Is your application a Unix process? Does it use files, tcp connections,
environment variables, standard Unix streams and command-line arguments
as inputs and outputs? Then Docker can run it.

Can your application's build be expressed as a sequence of such
commands? Then Docker can build it.

Escape dependency hell

A common problem for developers is the difficulty of managing all
their application's dependencies in a simple and automated way.

This is usually difficult for several reasons:

Cross-platform dependencies. Modern applications often depend on
a combination of system libraries and binaries, language-specific
packages, framework-specific modules, internal components
developed for another project, etc. These dependencies live in
different "worlds" and require different tools - these tools
typically don't work well with each other, requiring awkward
custom integrations.

Conflicting dependencies. Different applications may depend on
different versions of the same dependency. Packaging tools handle
these situations with various degrees of ease - but they all
handle them in different and incompatible ways, which again forces
the developer to do extra work.

Custom dependencies. A developer may need to prepare a custom
version of their application's dependency. Some packaging systems
can handle custom versions of a dependency, others can't - and all
of them handle it differently.

Docker solves the problem of dependency hell by giving the developer a simple
way to express all their application's dependencies in one place, while
streamlining the process of assembling them. If this makes you think of
XKCD 927, don't worry. Docker doesn't
replace your favorite packaging systems. It simply orchestrates
their use in a simple and repeatable way. How does it do that? With
layers.

Docker defines a build as running a sequence of Unix commands, one
after the other, in the same container. Build commands modify the
contents of the container (usually by installing new files on the
filesystem), the next command modifies it some more, etc. Since each
build command inherits the result of the previous commands, the
order in which the commands are executed expresses dependencies.

Note that Docker doesn't care how dependencies are built - as long
as they can be built by running a Unix command in a container.

Getting started

Docker can be installed on your local machine as well as servers - both
bare metal and virtualized. It is available as a binary on most modern
Linux systems, or as a VM on Windows, Mac and other systems.

How the project is run

Docker is a very, very active project. If you want to learn more about how it is run,
or want to get more involved, the best place to start is the project directory.

We are always open to suggestions on process improvements, and are always looking for more maintainers.

Legal

Brought to you courtesy of our legal counsel. For more context,
please see the "NOTICE" document in this repo.

Use and transfer of Docker may be subject to certain restrictions by the
United States and other governments.
It is your responsibility to ensure that your use and/or transfer does not
violate applicable laws.